<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
     .parent{
         width: 200px;
         background: orange;
     }
     .child{
         width: 100px;
         height: 100px;
         background: blue;
     }
    </style>
</head>
<body>
    <!-- 继承：子级对父级的关系 -->
    <!-- 特殊 -->
    <div class="parent">
        <div class="child">
        </div>
    </div>
</body>
</html>